Melt the butter in a saucepan, add the chopped onions, and cook until the onions are soft and translucent.
Add the chopped carrots, turnip and lentils and stir well into the onions. Pour in the stock.
Half cover the pan with a lid, and simmer gently for 35-40 minutes. Halfway through the simmering, add the chopped garlic.
Take the pan off the heat and cool slightly, then process in the blender.
Pour the smooth soup into a clean saucepan and reheat. Season to taste with salt and pepper.
Just before serving, stir the parsley through the soup